Norwegian communities as a rule have flags that
are banners of arms. There are very few exceptions, mostly for the
cities having traditional coats of arms (i.e.. old and non-heraldic,
according to the modern Norwegian heraldic practice). Among others,
such flags use Oslo, Trondheim
and Bergen.

While browsing though Norske by- og adelsvaben[trt33a]
I noticed an other such example - Drammen. Almost the same description is found in Norske
kommunevapen[c2j87]. My translation:

On 9 July 1930 the city government adopted the city flag to be a way
white stripe on a blue background.

This should represent the river Drammens-elven.

The Coat of Arms is based on the 1811 seal showing a column rising from a
stony base and over it in saltire a sword and a key. In 1933 book
this is shown golden on blue background, but the 1987 version shows
the charges silver on blue, I guess inspired from the colours of the
flag (this version of arms being adopted on 17 November 1960 after the
drawing of M.W. Hauerback, per [c2j87]).Željko Heimer, 5 July 2006

Actually the sword and key also appeared in silver earlier, for instance in
Haavin's Norske Byers Vaaben from 1898 [haa98],
so the flag is unlikely to be the source of the colouring. Traetteberg probably reduced the number of
colours to make the arms more heraldically correct. He was of the opinion
that arms should only have two tinctures, though many of the arms shown in
the 1933 book did not conform to his strict rules, but then his reform of
Norwegian civic heraldry had at that point not been carried through.Jan Oskar Engene, 6 July 2006

Would you believe I have nothing to add here that is not already on FOTW.
Interesting (or not), it seems that the municipal web site has no mention of the muncipal flag whatsoever
(at least that I could find in a quick search, maybe other
have better luck... not counting this and related
issue.)
One may also one to note a memorial pennant for the Drammen bridge
opening of 6 September 1936
here.
Also, the last page of the
document mention city flag to be used among
other in the various places in the city.
I am looking forward to this series of Norwegian municipal flags and I
only hope I shall find time to be able to keep up commenting on it.
Željko Heimer, 15 October 2015